The present invention relates generally to gas turbines, for example, for electrical power generation, and more particularly to cooling circuits for the first nozzle stage of a turbine.
The traditional approach for cooling turbine blades and nozzles is to extract high pressure cooling air from a source, for example, from the intermediate and last stages of the turbine compressor. A series of internal flow passages are typically used to achieve the desired mass flow objectives for cooling the turbine blades. In contrast, external piping is used to supply air to the nozzles, with air film cooling typically being used and the air exiting into the hot gas stream of the turbine. In advanced gas turbine designs, it has been recognized that the temperature of the hot gas flowing past the turbine components could be higher than the melting temperature of the metal. It is therefore necessary to establish a cooling scheme to more assuredly protect the hot gas path components during operation. Steam has been demonstrated to be a preferred cooling media for cooling gas turbine nozzles (stator vanes), particularly for combined-cycle plants. See, for example, U.S. Pat. No. 5,253,976, the disclosure of which is incorporated herein by this reference. However, because steam has a higher heat capacity than the combustion gas, it is inefficient to allow the coolant steam to mix with the hot gas stream. Consequently, it is desirable to maintain cooling steam inside the hot gas path components in a closed circuit. Certain areas of the components of the hot gas path, however, cannot practically be cooled with steam in a closed circuit. For example, the relatively thin structure of the trailing edges of the nozzle vanes effectively precludes steam cooling of those edges. Therefore, air cooling may be provided in the trailing edges of nozzle vanes. For a complete description of the steam cooled nozzles with air cooling along the trailing edge, reference is made to U.S. Pat. No. 5,634,766, the disclosure of which is incorporated herein by reference.
The present invention provides a cooling system for cooling the hot gas components of a nozzle stage of a gas turbine, in which closed circuit steam or air cooling and/or open circuit air cooling systems may be employed. In the closed circuit system, a plurality of nozzle vane segments are provided, each of which comprises one or more nozzle vanes extending between inner and outer walls. The vanes have a plurality of cavities in communication with compartments in the outer and inner walls for flowing cooling media in a closed circuit for cooling the outer and inner walls and the vanes per se. This closed circuit cooling system is substantially structurally similar to the steam cooling system described and illustrated in the prior referenced U.S. Pat. No. 5,634,766, with certain exceptions as noted below. Thus, cooling media is provided to a plenum in the outer wall of the segment for distribution therein and passage through impingement openings in a plate for impingement cooling of the outer wall surface of the segment. The spent impingement cooling media flows into leading edge and aft cavities extending radially through the vane. Return intermediate cooling cavities extend radially and lie between the leading edge and aft cavities. A separate trailing edge cavity may also be provided. The cooling media that flows through the leading edge and aft cavities flows into a plenum in the inner wall and through impingement openings in an impingement plate for impingement cooling of the inner wall of the segment. The spent impingement cooling media then flows through the intermediate return cavities for further cooling of the vane.
Impingement cooling is also provided in the leading and aft cavities of the first stage nozzle vane, as well as in the intermediate, return cavities of the vane. Inserts in the leading and aft cavities comprise sleeves having a collar at their inlet ends for connection with integrally cast flanges in the outer wall of the cavities and extend through the cavities spaced from the walls thereof. These inserts have impingement holes in opposition to the walls of the cavity whereby steam flowing into the inserts flows outwardly through the impingement holes for impingement cooling of the vane walls. Return or exit channels are provided along the inserts for channeling the spent impingement cooling steam. Similarly, inserts in the return intermediate cavities have impingement openings for flowing impingement cooling medium against the side walls of the vane. These inserts also have return or exit channels for collecting the spent impingement cooling steam and conducting it to the steam outlet.
As post impingement steam flow exits the aft cavities, it has conventionally experienced an expansion into the plenum-type cavity of the inner wall that is defined by the surface of the inner wall impingement plate. The impingement plate is curved to be disposed generally in parallel to the fillet region of the aerofoil. Thus, the impingement holes of the impingement plate in this region of the aerofoil fillet are oriented such that their center lines are perpendicular to the surface of the fillet. However, this also places many of these holes generally perpendicular to the flow exiting from the aft cavities. Accordingly, the problem exists that the cooling media, such as steam flow, exiting the aft cavities can adversely affect the performance of the steam cooling impingement holes in this region by creating an unstable, low static pressure steam supply to those holes.
The present invention was developed in particular for the purposes of steam cooling robustness in the area of the aerofoil fillet of the stage one nozzle.
The invention is thus embodied in structures that allow for the steam flow to exit the aft cavities in a manner which substantially isolates the same from the impingement holes in the vicinity of the exit of these cavities. This prevents the inner wall and aerofoil fillet impingement holes from receiving an unpredictable steam supply from the aft cavities.
The invention relates in particular to the configuration of the cavity insert and the flash rib configuration at the radially inner end of the first stage nozzle. More specifically, according to a first aspect of the invention, the invention is embodied in an extending flange or skirt to channel exit flow from the respective insert to isolate the same from impingement openings in the vicinity of the cavity exit ends. In a first embodiment, a flash rib boss is defined peripherally of at least one of the aft cavities and a flange or skirt extends radially inwardly from the boss. The skirt, which extends from the impingement boss, channels the flow exiting the corresponding aft vane cavity into the plenum radially inwardly of the impingement plate while shielding the impingement holes in the vicinity of that vane cavity from an adverse influence from the exiting steam flow.
In a second, alternate embodiment of the invention, the fin of the cavity insert for at least one of the aft cavities is extended in a radial direction, longitudinally of the insert so as to define a flange to channel the exit flow generally to an area beyond the fillet region and thereby substantially preclude an adverse effect on the impingement cooling in the vicinity of the cavity. Thus, in this embodiment, the fins of the cavity insert are extended to act as flow directing skirts which shield the impingement holes adjacent the cavity and the nozzle inner side wall.
A second aspect of the invention relates to the configuration of the interface between the cavity insert and the flash rib boss at the radially inner end of the first stage nozzle. More specifically, according to a second aspect of the invention, a gap between a flash rib or impingement boss, provided at the juncture of the impingement plate and the flash rib, and the cavity insert is controlled to minimize flow therebetween, so that flow out of the cavities is substantially limited to the flow out of the return or exit channel(s), where it will have a lesser impact on the impingement cooling of the aerofoil fillet region. In a presently preferred embodiment of the invention, the insert body defines a controlled gap with the flash rib boss irrespective of the location of the flange or skirt-like extension structure. The gap is most preferably controlled to about 0.02 inches.